When a party initiating a call to an incoming trunk hangs up, the incoming trunk is disconnected. When a trunk has been placed on hold, and the party at the far end of the trunk disconnects, the trunk is also dropped. This presents a significant problem to an operator of an emergency call system, referred to below as a 911 call system, because if a calling party in difficulty disconnects before the operator can answer the call, with the incoming trunk dropped, the operator has no way of knowing from what line the emergency call has been made.
If a call has been made either incoming, or outgoing from an operator, and the operator wishes to retain the emergency call while he summons an emergency service such as the police, the fire department, etc., he normally places the call on hold in order that he could use the same telephone instrument to call the emergency service. If the party in difficulty hangs up while on hold, the operator, in retrieving the call has no way of knowing whether the other party has hung up or is merely silent, for example due to some traumatic event.
For the above reasons, it is important for the operator to know whether the party in difficulty has in fact hung up, and preferably to know what was the identification of that incoming call. Without such information, it is not possible to adequately respond to certain types of emergency calls.